1. Technical Field
The present invention relates to systems and methods for controlling a vehicle powertrain.
2. Background Art
Vehicle manufacturers continually strive to improve fuel economy and reduce emissions while meeting customer expectations for performance and drivability. The availability of increasingly more powerful powertrain control computers has enabled more precise control of the vehicle powertrain and more efficient use of available energy to power the vehicle and related accessories. While the use of closed-loop or feedback control is generally preferred for lower feedgas emissions and best utilization of energy provided by conventional or alternative fuels, fuel cells, and/or batteries, the availability and accuracy of the sensors or estimators used to provide feedback to the controller may limit the operating conditions where closed-loop control is feasible. In addition, various compromises may be necessary to accommodate manufacturing and assembly variation and component wear over the vehicle lifetime of the vehicle.
For vehicle powertrains that include an internal combustion engine, either alone or in combination with other power sources as in a hybrid vehicle, accurate control of the combustion process is desirable to achieve emissions and fuel economy goals. To maintain stable combustion under varying engine, vehicle, and accessory operating conditions for desired fuel economy and emissions, the airflow and fuel supplied to the engine cylinders must be accurately controlled, particularly when the engine is operated at low load, such as during decelerations and/or idling. Precise airflow control is generally more problematic than fuel control under low load conditions. Conventional vehicles may operate in low-load conditions using closed-loop control of engine speed using airflow and spark to maintain stable combustion. The combustion stability limit may be defined in terms of airflow, or a corresponding engine torque value. Thus, a minimum engine torque may be set to ensure stable combustion. This torque value, also referred to as the “misfire torque limit,” is the lower bound of the engine torque production. In many vehicles, a buffer or error margin is provided such that the misfire torque limit is set above the true combustion stability limit. This buffer helps to ensure that the vehicle will not operate in the unstable combustion region, despite various factors such as throttle valve variability, engine friction losses that vary with temperature, variability in combustion efficiency associated with fuel variability, age and wear of the engine components, varying engine, vehicle, and ambient operating conditions, etc.
In a vehicle such as a hybrid electric vehicle (HEV) where the engine is operated in a torque control mode rather than an engine-speed control mode, the engine speed is generally independent of the combustion cylinder air mass and the spark ignition angle, and is therefore unsuitable for use in maintaining combustion stability. Excess torque produced by the engine in an HEV may translate into a charging current for the battery, which must be controlled to manage the battery performance and useful life. As such, it is desirable to set the misfire torque limit as accurately as possible while accommodating changes in the combustion stability limit from vehicle-to-vehicle as well as changes in operating conditions of a particular vehicle.
Systems and methods for adapting or adjusting the misfire torque limit to accommodate manufacturing variations and changing operating conditions are disclosed in US Pat. App. 2006/0025904, commonly owned by the assignee of the present invention. While suitable for many applications, the systems and methods disclosed adapt the misfire torque limit only under specific conditions that may require the engine to run longer or at higher speeds than otherwise required for current driving conditions. In addition, some extreme variations in operating conditions may not be accommodated if the specific entry conditions are not satisfied.